Nitrogen oxides (NOx) such as nitrogen monoxide and nitrogen dioxide are contained in various types of exhaust gas from factories, electric power facilities, other industrial facilities and motor vehicles. This NOx not only affects especially on the respiratory system of human body but also causes acid rain seen as a problem in global environment conservation. Therefore, a technical development effective for removing nitrogen oxides in various types of exhaust gas is desired.
Among methods for removing such nitrogen oxides known so far, there are a ternary catalytic method being used in exhaust gas treatment of a motor vehicle (a gasoline-powered vehicle) and a selective catalytic reduction method using ammonia as reducing agent. But the ternary catalytic method cannot apply to the exhaust gas containing oxygen in excess of theoretical amount necessary to completely oxidize an unburned hydrocarbon or carbon monoxide remaining in the exhaust gas.
Meanwhile, among methods for reducing/removing NOx from the exhaust gas containing excess oxygen known so far, there is a selective catalytic reduction method using a catalyst comprising V2O5—TiO2 and using ammonia as reducing agent. In this way, however, it is not easy to care for because of using highly smelling and harmful ammonia, and facilities grow in size because it needs a special apparatus for limiting emissions of unreacted ammonia. Therefore, it is not adequate to apply to a small-sized exhaust gas source or mobile source, and undesirable in economical efficiency.
In recent years, it has been reported that using an unburned hydrocarbon as a reducing agent remaining in the lean-burned combustion exhaust gas containing excess oxygen (ex. diesel engine) can accelerate a reduction reaction of NOx in the exhaust gas. Since this report, various catalysts to accelerate the reduction reaction of NOx have been developed, and many reports have been made, for example, alumina and transition metal-impregnated almina are effective for a reduction/removal reaction of NOx which uses hydrocarbons as reducing agent.
As an example of a catalyst which reduces/removes nitrogen oxides in the combustion exhaust gas containing excess oxygen with using such hydrocarbons as reducing agent, besides alumina and transition metal-impregnated alumina, a reduction catalyst which comprises alumina or silica-alumina including 0.1-4 weight % Cu, Fe, Cr, Zn, Ni or V of 0.1-4 has been reported (Refer to JP KOKAI Hei 04-284848).
Furthermore, it has been reported that the reduction reaction of NOx proceeds even in a low-temperature range of 200-300° C. when alumina impregnated with Pt and the like. (Refer to JP KOKAI Hei 04-267946, JP KOKAI Hei 05-68855 and JP KOKAI Hei 05-103949). However, the catalyst using such precious metals has defect that it is difficult to get selectively proceed a reduction reaction into harmless N2 because a combustion reaction of hydrocarbon which is a reducing agent is promoted excessively or because N2O which is seen as one of causative agent of global warming is side-produced in large quantity.
Furthermore, it is reported that alumina and the like impregnated with silver, with using hydrocarbon as reducing agent in the exhaust gas containing excess oxygen, makes NOx reductive reaction proceed selectively (Refer to JP KOKAI Hei 04-281844). After this report, lots of similar methods for NOx reduction/removal using the catalyst containing silver have been developed and reported (Refer to JP KOKAI Hei 04-354536).
However, each method for purifying the exhaust gas using NOx removal catalyst has a problem that, in the exhaust gas containing sulfur oxides and excess oxygen, the NOx removal performance deteriorates remarkably and the practicable durability is insufficient. Furthermore, it has also a problem that NOx removal performance is low if the majority of the exhaust gas is at relatively low temperature of 300-400° C.
Furthermore, the method for the NOx reduction/removal under coexistence of organic compounds which uses a hydrogenated zeolite catalyst or zeolite catalysts impregnated with V, Cr, Mn, Fe, Co, Ni and the like has been reported and, as the zeolite, Y-type zeolite, L-type zeolite, offretite-erionite mixed crystal-type-zeolite, ferrierite-type zeolite and ZSM-5-type zeolite are shown (Refer to JP Patent No. 2139645). Furthermore, the method for the NOx reduction/removal in the presence of methanol which uses a proton-type zeolite has also been reported and, as the zeolite, Y-type zeolite, ZMS-5-type-zeolite and mordenite are shown (Refer to JP Patent No. 2506598).
However, each method for the NOx reduction/removal which uses above-mentioned specific zeolite catalysts can not obtain the practicably satisfying NOx removal performance, and at present, has not been put into practical use.
An object of this invention is, in consideration of such circumstances, to provide a catalyst for purifying exhaust gas, having an excellent NOx removal performance and durability even against the exhaust gas containing sulfur oxides, and having a high denitration performance and an excellent practicality even when the exhaust gas is at relatively low temperature of 300-400° C., and to provide a method of purifying exhaust gas using the catlyst.